A student writes:

  Dr. Patt:

  My name is *********** and I am in your EE 306 class.  

  I had a question about problem #15 in the homework.  I understand 
  everything about the question except the starting conditions.  Do 
  we assume all the machines are busy or all inactive?  Or do we 
  start with the ending situation from Example 2.5 in the book?  The 
  problem states: "Refer to Example 2.5 for the following questions."  

  I do not know what bit pattern to start out with, and then begin 
  manipulating with the masks.  Also, after finishing part a, do we 
  use the existing pattern in part b, or do we go back to the beginning 
  pattern?   Please clarify this for me.  Thanks!  If this is not clear 
  enough, please email me back, and I will elaborate more.  Thanks again.

  <<name deleted to protect the guilty>>

Thank you for the email.  Yes, your question is clear.
And, you did the right thing -- reading Example 2.5 first.

The question asks you about the mask and operation.  This is independent 
of the starting condition of the bit vector.

That is, the mask MUST and operation must produce the desired result 
REGARDLESS what the bit vector is BEFORE applying the mask + operation.

For example, if the bit vector is  [a b c d e f g h] beforehand, after 
we perform the mask + operation to show machine 2 is busy, the bit 
vector will look like [a b c d e 0 g h], where a,b,c,d,e,f,g,h are all 
binary values (i.e., 0 or 1).

Got it?

Yale Patt